Process for acid pickling



Oct. 27, 1970 u. T. HILL ErAL 3,536,601

PRocEss FOR ACID PICKLING v Filed March 7, 1968 United States Patent 3,536,601 PROCESS FOR ACID PICKLING Uno T. Hill, Gary, and Irwin Rosenak, Munster, Ind., assignors to Inland Steel Company, Chicago, Ill., a corporation of Delaware Filed Mar. 7, 1968, Ser. No. 711,252 Int. Cl. B08b 3/08; (228i` 13/.00

U.S. Cl. 204-147 3 Claims ABSTRACT F THE DISCLOSURE This invention relates to a process and apparatus for pickling of ferrous metals and alloys. More particularly, it relates to an improved pickling process and apparatus which permits controlling the extent of pickling achieved in the continuous pickling of ferrous metals and alloys regardless of variations of speed of the product through the acid pickling bath.

In the pickling of ferrous metal and alloy products, it is desirable to limit the extent of acid attack which occurs on the metal after the surface layer of oxides has been removed. It is conventional in pickling operations to include for this purpose in the pickling bath small concentrations of chemical inhibiting agents, such as amines, phosphates, suldes or colloidal materials. The use of chemical inhibiting agents, however, has certain disadvantages, particularly in continuous pickling operations. Many of the chemical inhibitors react with the metal to produce undesirable compounds which are contaminants in subsequent processing of the metal. Further, at the relatively high line speeds at which continuous pickling operations are designed to operate, chemical inhibitors have an undesirable retarding effect on the pickling process. If the inhibitor is present in suicient quantity to give a desirable inhibiting effect in the event that the line speed drops substantially, it at the same time inhibits the pickling process when the line is operating at its design rate of speed. Accordingly, in order to achieve high line speeds, the concentration of chemical inhibitor in the pickling bath must be kept to a minimum. Under these circumstances, however, difficulties arise when for any reason it is necessary or desirable to stop or substantially reduce the speed of the line. When such slowdown or stoppage occurs, the section of the metal in contact with the acid bath is excessively attacked by the acid and for this reason may have to be discarded.

In the design of continuous pickling lines for treatment of steel strip, it is conventional to provide for a constant high speed through the line. In order to maintain a continuous supply of strip through the line, it is also conventional to provide duplicate holders for coils of the strip at each end of the line. In order to avoid stopping the line to switch from one roll or coil of strip to another, it is conventional to make provision for storing strip in the line during the time required for switching. It is usual to provide this storage by means of one or more looping pits, `free hanging loops, looping towersl or looping cars at each end of the line. In addition to the initial expense of construction, the use of looping cars, towers, or pits in this manner also has certain inherent disadvantages, in that the strip can be damaged by scratching in the storage facility resulting in a loss of yield or deterioration in quality. In addition, the rela- Mice tively severe bending of the material which occurs in its passage through such devices tends to cause undesirable work hardening of the strip.

In accordance with the invention, the disadvantages which have heretofore existed in the pickling of ferrous metals and alloys, particularly in continuous pickling of steel strip, is avoided Vby means of an electrical inhibition system for the acid bath which is correlated to the speed of the line, so as to give maximum inhibition when the `strip is stopped in the acid bath or moves only very slowly and to `give little or no inhibitions when the strip is passing through the pickling bath at its normal high speed. Since the line can be stopped with no damage to the material remaining in the acid bath it is unnecessary to provide the usual storage space necessary for switching feed coils and therefore the disadvantages and diiiculties inherent in such storage space are avoided when the process of the invention is used.

In accordance With the invention, a variable DC voltage is impressed between the acid bath and the strip passing therethrough, with a polarity such that the strip is negative with respect to the acid solution. The magnitude of the DC voltage is correlated to the speed of the line so that the amount of inhibiting effect produced by the voltage is a minimum (or zero) when the line is traveling at a selected high speed and is a maximum value suflicient to prevent undesired acid attack when the strip is stopped in the acid solution for any reason.

The invention will be better understood from the following detailed description thereof, taken in conjunction with the accompanying drawings, in which FIG. 1 is a top view in schematic form of a typical continuous line for the pickling of steel strip, in which provision for carrying out the invention has been made; and

FIG. 2 is a side View of the same pickling line shown in FIG. l.

Referring to FIGS. 1 and 2, much of the apparatus used in carrying out the process of the invention is conventional in form. As shown, a coil 10 of steel strip 15 to be treated is mounted on payoff reel 10a at the feed end of the line. The strip 15 passes through pairs of pinch rolls 11 and 12 which are used to assist in moving the strip and maintaining the proper tension thereon. Between pinch rolls 11 and 12, is a conventional strip joiner 13 used to connect consecutive rolls of strip by stitching or welding. Elongated acid tank 14 contains an acid bath 14a comprising any of the mineral acids, such as sulfuric, nitric, or hydrochloric, conventionally used for pickling iron and steel products. The length of tank 14 is correlated with the normal speed of the strip 1S passing therethrough, as well as with the concentration and temperature of the acid bath, as will be apparent to those skilled in the art, so as to achieve the desired degree of pickling during normal operation of the line. Although only one acid tank 14 is shown in the drawings, it should be understood that more than one may be used as necessary to accomplish the desired pickling. In its passage through acid tank 14, strip 15 is supported at each end of the tank by rolls 19 and 19a. At least one of rolls 19 and 19a is made electrically conducting, and, being in contact with strip 15, transmits to the strip any voltage applied to the roll.

Leaving acid tank 14, the roll 15 passes through pairs of pinch rolls 20 and 21, which guide the strip in its passage through rinse tank 18 in which the acid bath remaining on the strip is removed, as by suitable sprays (not shown) or by the contact with a rinsing solution contained within the tank.

Continuing on its route, strip 15 passes through dryer 22 (shown schematically), in which the strip is dried, and then through pinch rolls 23, shear 26, support roll 27 and is wound as a coil 28 on tension reel 28a in a conventional manner.

Attached to pinch rolls 23 is a tachometer 24 (not shown in FIG. 2) which measures the speed of the strip 15 passing through the line and supplies an instantaneous control signal to voltage controller 31, which in turn emits a DC voltage which is inversely proportional to the speed as measured by tachometer 24. This DC voltage is impressed by lines 32 and 33 between electrodes 34 and electrically conducting roll 19u.

The output of voltage controller 31 has a polarity such that a positive voltage appears on conductor 32 while a negative voltage is present on conductor 33. In this way, electrodes 34 which are positioned to make electrical contact with solution 14a in acid tank 14 are made positive with respect to roll 19a. Electrodes 34 are suitably made of graphite or carbon or any other electrically conducting materials which will not be adversely affected by contact with the acid solution within tank 14. Since strip 15 is in contact with roll 19a, the same voltage difference exists between strip 15a and acid solution 14a in contact with electrodes 34, and the strip 1S is therefore made negative with respect to the acid solution.

Voltage controller 31, shown schematically in the drawing, is a known commercially available device which produces a variable DC output voltage in response to a suitable control signal, in this case supplied by tachometer 24.

The output voltage from controller 31 varies inversely with the speed of strip 15 as measured by tachometer 24. In general, as the speed of strip 15 decreases, the output voltage and thus the degree of inhibition of acid bath 14a increases. An inverse proportional (linear) relationship between the speed and the voltage will generally be satisfactory, although other arrangements, e.g., logarithmic, may also be found advantageous and can be used in the invention.

In accordance with the invention, as the speed of strip 15 increases, the output voltage from controller 31 decreases in some prearranged manner, as described above, until at the maximum design speed contemplated for operation of the line voltage controller 31 produces either a very low inhibiting voltage or none at all. On the other hand, if for any reason the line is stopped, voltage controller 31 applies a voltage between strip 15 and acid solution 14a which is suiciently large to inhibit the strip in contact with the solution against further acid attack for the period during which the line is stopped.

It will be apparent to those skilled in the art that the absolute magnitude of the voltage emitted by controller 31 is a function of the extent of inhibition necessary to protect the strip during periods when it is stationary in the acid tank. This in turn depends on the nature and concentration of the acid used for pickling and the temperature and other variables, of the pickling operation, as will be apparent to those skilled in the art. The important consideration is that the voltage controller must produce a suciently large voltage to protect a stationary strip within the acid tank for any desired period of time.

Although in FIG. 1, the connection between line 33 (the negative conductor from controller 31) is shown connected to roll 19a, it should be appreciated that this connection could also have been made either to roll 19 or to both of rolls 19a and 19. In FIG. 1 there is also shown a connection 33a between conductor 33 and the tank 14. This isa generally desirable expedient which will inhibit attack of the acid on the acid tank itself, assuming that the tank is made of a corrodible metal. It is not a necessary aspect of the invention.

Although as previously mentioned, the absolute magnitude of the inhibiting voltage is dependent on operating conditions, it will be found that the necessary voltage is not large. A suitable voltage difference of less than about 5-6 volts between the acid solution and the strip will be found to give suitable protection even during periods when the strip is stationary within the tank, although under eX- treme conditions higher voltages may be found necessary.

vThe foregoing detailed description has -been given for clearness of understanding only and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

What is claimed is:

1. In the pickling of a ferrous metal or alloy product wherein said product is immersed in and moved through an acid pickling bath at a speed which can very between a minimum and a maximum value, the improvement comprising controlling the degree of acid attack on said product by impressing a DC voltage between said product and said acid bath, said product being negative with respect to said acid bath, and correlating the magnitude of said impressed voltage inversely with the speed of said product through said bath so as to provide a relatively high voltage when the speed is at its minimum value and a relatively low voltage when the speed is at its maximum value, whereby substantially uniform pickling of said product is achieved regardless of variations in said speed.

2. The method of claim 1 wherein said product is an elongated steel strip.

3. The method of claim 2 wherein said strip passes continuously through said bath at a variable speed and said impressed voltage is inversely proportional to the speed of said strip.

References Cited UNITED STATES PATENTS 2,404,948 7/1946 Croco 204-211 3,022,234 2/ 1962 Anderson 204-196 3,081,251 3/1963 Spector 204-196 3,303,118 2/1967 Anderson 204-196 3,473,962 10/1969 Hampel 134-32 TA-HSUNG TUNG, Primary Examiner U.S. Cl. X.R. 

